Current limiting device



Feb. 25, 1964 R. R. ROALEF 3,122,654

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CURRENT LIMITING DEVICE Original Filed May 15, 1958 5 Sheets-Sheet 5CURREh POTENTIAL AT TERMINAL 13 IN V EN TOR.

ROBERT R. ROA EF f BY W ATToRNa f United States Patent 3,122,654 CURRENTLlMlTlNG DEVICE litobert R. Roalef, 321 Kenilworth Ave., Dayton, OhioOriginal application May 15, 1958, er. No. 735,651, now

Patent No. 3,027,466, dated Mar. 27, 1962. Divided and this applicationNov. 28, 1961, Ser. No. 155,512

3 Claims. (Cl. 307-885) (Granted under Title 35, U.S. Code (1952), see.266) The invention described herein may be manufactured and used by orfor the United States Goverment for governmental purposes without thepayment to me of any royalty thereon.

This application is a division of my application Serial 21606. 735,651,filed May 15, 1958, now Patent No. 3,027,-

This invention relates to current limiting devices and is primarilyconcerned with the protection of delicate current sensing apparatus,such as sensitive null meters or the sensitive error signal sensingamplifiers used in servo systerns, against overloads and consequent burnout.

The ideal protective device would otter no resistance, or very littleresistance, to current flow to the protected apparatus at low appliedvoltages, so as to preserve the sensitivity of the apparatus to weaksignals, and would act automatically at higher applied voltages to limitthe current to the protected apparatus to a safe value. The currentlimiting devices described herein utilize the peculiar properties ofsemiconductor diodes to approach this ideal.

A more detailed description of the invention will be given withreference to the specific embodiments thereof shown in the accompanyingdrawing in which FIG. 1 illustrates the characteristic of a diode havinglow incremental impedance at zero voltage, such as a germanium diode,

FIG. 2 is a current limiter employing diodes having characteristics asshown in FIG. 1,

FIG. 3 shows the characteristic of the limiter of FIG. 2,

FIG. 4 shows a limiter employing series diodes of the type illustratedin FIG. 1 and shunt diodes of the type illustrated in FIG. 5,

FIG. 5 illustrates the characteristic of a diode having high incrementalimpedance at zero voltage, such as a silicon diode.

F IG. 6 illustrates the characteristic of FIG. 4,

FIGS. 7, 8 and 9 illustrate the principles involved in a current limiterhaving an adjustable threshold and using diodes of either the type shownin FIG. 1 or in FIG. 5,

FIG. 10 is a limiter circuit employing the principles of FIGS. 7, 8 and9,

FIG. 11 is the overall characteristic of the limiter in FIG. 10.

FIGS. 12a and 12b illustrate the operation of FIG. 10,

FIGS. 13 and 14 are measured characteristics of limiters of the typeshown in FIG. 10,

FIG. 15 illustrates a current limiter requiring a smaller biasingvoltage than the limiter of FIG. 10,

FIG. 16 is the characteristic of the limiter in FIG. 15,

FIGS. 17 and 18 illustrate alternating current biasing in circuits ofthe type shown in FIGS. 10 and 15,

FIGS. 19 and 20 illustrate limiters of the type shown in FIGS. 10 and 15used as phase detectors,

FIG. 21 illustrates the phase characteristic of the circuits of FIGS. 19and 20.

FIG. 1 shows the E-I characteristic of a typical junction type germaniumdiode such as the General Electric IN93. For current flow in the backdirection, the impedance of the diode at C. is seen to be relatively lowin the vicinity of zero current, as indicated by the steepness of thecharacteristic, and to increase abruptly to a relatively high value atabout .1 volt and 23 microamperes. At 40 C. this change in impedanceoccurs at between .1 and .2

volt and 70-80 microamperes. The points are representative and can beexpected to vary somewhat from sample to sample.

Two diodes having characteristics of the type shown in FIG. 1 may beconnected in oppositely poled series relationship to form a simplecurrent limiting device, as shown in FIG. 2. Here a current limiterhaving terminals 1 and 2 and consisting of oppositely poled germaniumdiodes 3 and 4 is connected in series with coil M, which, for example,may be the coil of a null meter or the input coil of a magneticamplifier, and serves to protect the coil against excessive current flowin either direction. The characteristic of the circuit of FIG. 2 is asshown in PEG. 3. For current how in the direction shown, diode 3 ottersa constant relatively low impedance for all values of e. Diode 4 ottersa similar relatively low impedance for values of e below the value,varying with temperature, that brings the diode voltage to the point ofinflection on the diode characteristic. When the current flow is in theopposite direction the roles of diodes 3 and 4 are reversed. For smallvalues of e, therefore, the combined impedance of diodes 3 and 4 isrelatively low and is less than the imedance of coil M, so that thesensitivity of the protected apparatus to small voltages is not greatlyreduced by the current limiting device. For higher values of 2, abovethat at which inflection occurs, the greatly increased impedance ofdiode 3 or 4 prevents any appreciable further increase in currentthrough the coil M. This type of protective device is simple and can beinstalled within the case of a meter, thereby permitting a sensitivemeter to be used in circuits Where overloading is likely to occurwithout the danger of meter burn out.

As indicated, the protection afiorded by the above described device isnot independent of temperature, which may possibly result in inadequateprotection being provided at higher temperatures. The arrangement ofPI". 4 is intended to alleviate this ditficulty through the use of apair of oppositely poled silicon diodes 5 and 6 in shunt to coil M. TheE-I characteristic of a typical point contact silicon diode, such as theHughes No. 6002, is shown in FIG. 5 and is substantially independent oftemperature. It is seen that the diode has a very high impedance,substantially an open circuit, in the back direction and also a highimpedance in the forward direction up to a voltage between .3 and .5volt where there is a sharp inflection of the curve representing anabrupt drop in impedance to a relatively low value. In the arrangementof FIG. 4, therefore, I as seen in FIG. 6, increases as e increases upto a value e at which the voltage across the coil M and the silicondiodes is somewhere between .4 and .5 volt and the impedance of theforward connected diode starts changing to its lower value. At thispoint the diode begins to conduct so that the total current I dividesbetween coil M and the shunting diode. The eifect of this is to limitfurther increases in voltage across the coil and in I as shown in FIG.6. Since the silicon diode characteristics are substantially unaifectedby temperature changes, the shunting diodes protect the coil against theincreased current permitted by the germanium diodes at the highertemperatures.

Because of the fact that silicon diodes require a forward voltage ofabout .5 volt to produce an abrupt fall in impedance, as shown in FIG.5, these diodes are not suitable for use in a simple limiting circuit ofthe type shown in FIG. 2. However, by the use of a direct biasingvoltage in circuit with a pair of silicon diodes, it is possible toconstruct a limiting circuit having a characteristic similar to thecircuit of FIG. 2 and providing the advantage that, as a result of usingsilicon diodes, the limiter characteristic is substantially independentof temperature. FIGS. 7-1-2 illustrate the principles underlying thedesign of this circuit.

:Referring to FIG. 7, silicon diode 7 is connected in series with asource of direct voltage E and a resistance R. A source 8 of lowinternal impedance relative to R and providing a variable direct voltage2 is connected across the diode. It is apparent that I :I I FIG. 8 showsthe manner in which I I and their algebraic difference I vary with e. Ase decreases from a value above E which is the voltage that would existacross the diode if source 8 were removed, I increases and I decreases,becoming equal and reducing I to zero at e=E At this point I =I =I whichis referred to as the quiescent current. As e decreases below E Ireverses its direction and flows into source 8. This current begins tolimit as e enters the region of flexure of the I curve. Below thisregion the diode has a very high impedance, as indicated by the flatnessof the I curve, and the value of I is determined principally by R which,as stated above, is assumed to be much greater than the internalimpedance of source 8. In FIG. 8 the internal impedance of source 8 isassumed to be substantially zero so that E e I s R for values of 2 belowthe flexure region. The resistance presented by the circuit to source 8at e=E is very low as indicated by the steepness of the e-Icharacteristic at this point. By increasing R to a relatively high valueand then increasing E, as required to reestablish the same value of Ithe limiting action of the circuit for current flow into source 8 can beimproved, as indicated in FIG. 9 where that part of the I curve belowthe fiexure region is much flatter than in FIG. 8.

It will be seen that the shape of the e-I characteristic of FIG. 9 isvery similar to that of the EFI characteristic of silicon shown in FIG.5. Also, if the point E is considered the origin, the characteristic isvery similar to that of germanium shown in FIG. 1. By using a seconddiode 8 and duplicating the circuit of FIG. 7, as shown in FIG. 10, Ecan effectively be made the origin with respect to an applied voltage eof either polarity. Therefore, the circuit between terminals 1 and 2acts as a current limiting device for coil M and, as shown in FIG. 11,has a characteristic which is similar to the characteristic of thecircuit of FIG. 2. Figs. 12a and 12b illustrate the operation of thecircuit of FIG. 10. For small values of e, of the polarity shown, thecurrent I travels through diode 7 in the forward direction and throughdiode 8 in the backward direction, the impedance of the diodes beingvery low relative to R for voltages in the vicinity of E as indicated inFIG. 9 by the steepness of the character istic at this point. For valuesof e causing the voltage across diode 8, as measured from E in FIG. 9,to go beyond the fiexure region of its characteristic, the diodepresents substantially an open circuit to the current so that it mustflow through the battery and R, as shown in FIG. 12!), and is limited tothe value permitted by the magnitude of this resistance. Due to thesymmetry of the circuit, reversing the polarity of e merely interchangesthe roles of diodes 7 and 8.

As may be seen by a comparison of FIGS. 8 and 9, the effectiveness ofthe circuit of FIG. as a current limiter increases as R increases. Foreffective limiting, as indicated by a low value of slope for that partof the e-I curve corresponding to values of e below the flexure region,a relatively large R is required. FIGS. 13 and 14 show the measuredcharacteristics of the circuit of FIG. 10 for two values of R with thevalues of E required to produce limiting in the 35 microarnpere region.FIG. 13 shows the effect of the coil M or load resistance on one of thecharacteristics for low values of e. It is apparent that limiting takesplace when the current reaches a value equal to the quiescent current Iwhich is directly related to E/R and, for large values of R, issubstantially equal to E/R, as seen in FIGS. 8 and 9. Therefore, for anygiven value of R, the limiting point can be controlled by t. changing E.For quiescent currents in the microampere range, as would occur whenprotecting a very sensitive meter element, it is feasible to use smallbatteries to supply the voltage E since at this low current drain thebattery life can be expected to approximate shelf life. The voltage Ecan of course be obtained from a conventional DC. power supply insteadof a battery where complete portability is not required.

As seen from FIGS. 13 and 1-4, for efiective limiting at currents of 30microamperes or greater, relatively high values of E are required, whichmay be undesirable where batteries are used to supply the potential. Thecircuit of FIG. 15 provides absolute limitation of the current throughcoil M over a wide range without requiring high direct biasing voltages.Considering this circuit further and assuming 15:0, battery 9 causes acirculating current 1 to flow in the loop circuit consisting of battery9, resistor 10, diode D and coil M. Similarly, battery -11 causes acirculating current I to flow in the loop circuit consisting of battery11, resistor 12, diode D and coil M. If'the sources 9 and 11 provideequal voltages E, if resistors 10 and 12 have equal values R and if thediodes D and D are matched, then I =I and, since the two currents flowin opposite directions through coil M, the current I is zero. Diodes Dand D may be of germanium or silicon, but are preferably the latter inorder to take advantage of the relative independence of temperatureexhibited by the silicon diode characteristic.

The voltages E bias diodes -D and D in their conductive regions at avoltage at which the incremental impedance is low. This biasing voltageacross the diode terminals may be designated E and may have, forsilicon, a value of .51 volt, for example, giving a diode current ofabout 35 microamperes as seen in FIG. 5. The value of R is determined bythe relationship E-E n and for the above specific case, using a smalllow voltage battery such, for example, as the Mallory PR-l mercury cellhaving a potential of 2.6 volts,

= 59,700 ohms to a certain value E the voltage across D is such thatits.

incremental impedance is low. For the diode of FIG. 5, E would be thatvalue of e for which the voltage across the diode is reduced to about.48 volt. Over this range of e, I =I As 2 increases above E theincremental impedance of D begins to increase, as indicated by thedecreasing slope of the characteristic in FIG. 5, and increases rapidlyuntil the potential across the diode is reduced to about .3 volt atwhich point D represents substantially an open circuit. As the impedanceof D in creases an increasing portion of the increment in I flowsthrough resistor 12 until eventually any significant further increase inthe current through D and therefore.

through M, is impossible. Since the current through D equals I I whenthis current becomes zero [3 :1 so that the current in coil M positivelylimits at a value equal to the quiescent current. As e increases stillfurther I remains constant at the quiescent value while I continues toincrease at a rate determined by R. This is illustrated in FIG. 16. Dueto the symmetry of the circuit, reversing the polarity of e simplyinterchanges the roles of D and D and reverses the direction of I Asstated above, the current at which the circuit of FIG. 15 limits isequal to the quiescent current I Since this current is determined by Eand R in accordance with the relation the limiting value can be variedby appropriate changes in E or R. For limiting in the range of 30-80micro amperes with a low value of E, the values of R are relatively lowand the circuit, when in the limiting state, does not present as high animpedance to the source of e as is presented by the circuit of FIG. 10.However, by mcreasing E and R in proportion any desired circuitimpedance may be obtained for any given limiting current. E is nearly aconstant quantity as may be seen in FIG. where over a current range of-80 microamperes it varies less than .1 volt. As in FIG. 10, the smallcurrent drain permits the life of the batteries supplying E toapproximate their shelf life. E can, of course, be obtained from aconventional D.C. power supply if desired.

It is not necessary that the circuits of FIGS. 10 and be biased withdirect current. Alternating biasing voltages may be used as shown inFIGS. 17 and 18. In FIG. 18, the phasing of the transformer secondarywindings must be as indicated and the secondary voltages must be equal.The operation is similar to the direct current biased condition exceptthat a pulsing rather than a continuous direct current passes throughcoil M.

The signal e in the above case may be alternating as well as directprovided it has the same frequency as E and provided there is a fixedphase difference, preferably 0 or 180, between e and E The smallestalternating voltages may be measured since the diodes are forced intoconduction by the biasing voltage once in each cycle and whileconductive the smallest departure of e from Zero can cause a currentflow through M. It is, therefore, not necessary for the signal beingmeasured to provide the initial voltage required to cause diodeconduction. It is evident that changing the phase relation between 2 andE by 180 reverses the direction of current flow through M. The circuitmay therefore be used as a phase sensitive rectifier in servo systems inwhich the error signal is an alternating voltage having one of twoopposite phases depending upon the direction of the error. In thisapplication, e would represent the error signal and E would supply thereference phase. A reversal in phase of e, indicating a change in thedirection or sign of the error, results in a change in direction of thedirect current in coil M which may be the input to a sensitive servocontrol system. In this case also, the limiting action of the circuitprotects coil M against high values of the error signal e. Where theabove 0-180 relationship between the two applied alternating voltages isnot maintained, the output magnitude is not indicative of error signalmagnitude; however, the output polarity is still indicative of phaserelation, in this case indicating the direction of departure of onevoltage from a quadrature relation to the other.

The circuits of FIGS. 17 and 18 also may be used to give a directcurrent indication of the phase relation between two alternatingvoltages of the same frequency which is substantially independent of therelative magnitudes of the two alternating voltages. FIGS. 19 andillustrate such use of the circuits of FIGS. 17 and 18, respectively.Referring to FIG. 19, this circuit compares the phase of 2 with that ofE which may be considered the reference phase, and produces a positivepotential at terminal 13 when the phase difference is in the range 0 90and a negative potential when in the range 90180, the potential beingZero at 90. The characteristic is illustrated in FIG. 21. E correspondsto the biasing voltage E of FIG. 10 and is selected in accordance withthe maximum voltage desired at terminal 13. The signal 2 must havesufiicient magnitude in this case to drive the circuit into its limitingrange. In other words, e must exceed the magnitude required at phaseangles 0 or 180 to reduce the potential across the diode where itopposes the biasing potential sufficiently to drive that diode into itshigh impedance range. If the diodes are biased at about the pointindicated in FIG. 5 the required minimum potential reduction would beabout .1 volt or slightly more.

The operating principle of the circuit of FIG. 19 is no different fromthat of the circuit of FIG. 10. If the phase difference between 2 and Eis 0 these two voltages have the same phase at diode 7 and oppositephases at diode 8. Therefore, on the positive half-cycles of E diode 7has a low impedance and diode 8 a very high impedance so that currentflows from the secondary of transformer 14 through diode 7, thesecondary of transformer 15, resistor 16 and resistor 17 causingterminal 13 to be positive relative to ground. If the phase differenceis 180 the two voltages are in phase on diode 8 and of opposite phase ondiode 7. Consequently, the previous conditions are reversed with currentflowing from the secondary of transformer 14 upward through resistor 17and thence though diode S, the secondary of transformer 15 and resistor18, causing terminal 13 to be negative. When the phase difference isequal currents flow in opposite directions through resistor 17 duringeach positive halfcycle of E so that the net current and the voltage atterminal 13 are zero. As in FIG. 10, the limiting action of the circuit,resulting from the high value of R, makes the voltage at terminal 13substantially independent of the magnitude of e When operated under theconditions specified above for FIG. 19, the circuit of FIG. 20 operatesto produce the same result, the basic operating principle being the sameas for FIG. 15. When 2 and E have a phase difference of Zero, the twovoltages at D due to e, and E have the same phase whereas the twoVoltages at D have opposite phases. D therefore has a very highimpedance, provided e exceeds the minimum value specified above asrequired, and the current from transformer 14- flows through D and thendivides, part flowing through resistor 17 and D to the extent requiredto cancel the opposite fiow through D due to transformer 15", and theremainder flowing through the secondary of transformer 15" and resistor12. Terminal 13 is therefore positive. If the phase dilference isconditions are exactly reversed so that current flows upward throughresistor 17 toward terminal 13 with the result that this terminal isnegative. When the phase difference is 90, equal currents flow inopposite directions through resistor 17 during each positive half-cycleof E with the result that the net current and the voltage of terminal 13are zero. Since the current through resistor 17 can never exceed thequiescent current through D or D resulting from the biasing voltagesfrom transformers 15' and 15", the potential at ter minal 13 isindependent of the magnitude of 2 in the range above the specifiedminimum. The maximum voltages at terminal 13 can be adjusted by changingR or the equal secondary voltages of transformers 15' and 15".

I claim? 1. A phase sensitive rectifier for small error signalscomprising: a pair of input terminals a and b for receiving saidalternating error signal, a pair of output terminals c and d, asemiconductor diode connected between terminals a and c, a semiconductordiode connected between terminals b and d, said diodes having like polesconnected to said output terminals, a source of alternating referencevoltage and an impedance connected in series between terminals a and dand a like source of alternating reference voltage and a like impedanceconnected between b and c, said reference voltage sources being poledand phased to send in-phase alternating currents through said diodes inthe forward direction, said error signal being of the same frequency assaid reference voltages.

2. Apparatus as claimed in claim 1 in which said error signal has one ofthe two phase angles 0 and 180 relative to said reference voltages.

3. Apparatus for producing a direct current output the magnitude ofwhich is related to the deviation of the phase of a first alternatingvoltage from a quadrature relation to the phase of a second alternatingvoltage of the same frequency and the polarity of which is indicative ofthe direction of the deviation, said apparatus comprising: a pair ofinput terminals a and b for receiving said first alternating voltage, apair of output terminals 0 and d, a semiconductor diode connectedbetween terminals a and c, a semiconductor diode connected betweenterminals b and d, said diodes having like poles connected to saidoutput terminals, means for dividing said second alternating voltageinto equal com- 1 2,618,753

ponents, means for connecting one component in series with an impedancebetween terminals a and d and the other component in series with a likeimpedance between terminals b and c, said components being poled andphased to send in-phase currents in the forward direction through saiddiodes, the magnitude of said first voltage exceeding that required,when the phase angle between said first and second voltages has one ofthe voltages 0 and 180, to reduce the voltage across one of said diodesto the point where its impedance is 'high relative to the aforementionedimpedances.

References Cited in the file of this patent UNITED STATES PATENTS VanMierlo Nov. 18, 1952

1. A PHASE SENSITIVE RECTIFIER FOR SMALL ERROR SIGNALS COMPRISING: APAIR OF INPUT TERMINALS "A" AND "B" FOR RECEIVING SAID ALTERNATING ERRORSIGNAL, A PAIR OF OUTPUT TERMINALS "C" AND "D," A SEMICONDUCTOR DIODECONNECTED BETWEEN TERMINALS "A" AND "C," A SEMICONDUCTOR DIODE CONNECTEDBETWEEN TERMINALS "B" AND "D," SAID DIODES HAVING LIKE POLES CONNECTEDTO SAID OUTPUT TERMINALS, A SOURCE OF ALTERNATING REFERENCE VOLTAGE ANDAN IMPEDANCE CONNECTED IN SERIES BETWEEN TERMINALS "A" AND "D" AND ALIKE SOURCE OF ALTERNATING REFERENCE VOLTAGE AND A LIKE IMPEDANCECONNECTED BETWEEN "B" AND "C," SAID REFERENCE VOLTAGE SOURCES BEINGPOLED AND PHASED TO SEND IN-PHASE ALTERNATING CURRENTS THROUGH SAIDDIODES IN THE FORWARD DIRECTION, SAID ERROR SIGNAL BEING OF THE SAMEFREQUENCY AS SAID REFERENCE VOLTAGES.